1. Field of the Invention
The present invention relates to a pneumatic tire provided with a belt arranged in such a manner that cords are inclined with respect to a tire circumferential direction.
2. Description of the Related Art
As an internal structure of a pneumatic tire used in a heavy load vehicle such as a truck, a bus or the like, there has conventionally known a structure in which a reinforcing belt is arranged in an outer peripheral side of a carcass forming a framework of the tire. The belt is generally arranged in such a manner that cords are inclined at a predetermined angle with respect to the tire circumferential direction, and a cord angle is uniform over an entire tire width direction.
However, there is a case that the cord angle is unevenly changed by going through a curing step (Japanese Unexamined Patent Publication No. 2007-84035). This means that the cord angle of the belt with respect to the tire circumferential direction becomes larger in both end portions than in a center portion in the tire width direction, in correspondence to an operation expansion changing the tire so as to bring the tire into close contact with a mold. Accordingly, an end number (a cord number per unit width) becomes smaller and a constraint force tends to be lowered, in both end portions of the belt.
FIG. 7 is a conceptual diagram showing the belt in which the cord angle is changed as mentioned above. Reference symbol C denotes a tire equator line, and reference symbol W denotes a belt width (a half of the belt is illustrated). A broken line is a virtual line in the case that the cord angle is uniform. A cord angle θ with respect to the tire circumferential direction becomes larger in proportion as being away from the tire equator line C, and becomes maximum at an end portion of the belt. Accordingly, the constraint force of the belt becomes relatively low in the end portion rather than in the center portion.
A pneumatic tire is changed in such a direction that a diameter becomes large, by filling an internal pressure or traveling again and again. This change is called as a diameter growth. If the diameter growth makes progress, a form retention of the tire is deteriorated, and a uniformity is lowered, thereby causing an irregular wear. Accordingly, it is desirable to make the diameter growth as small as possible, and it is important to secure the constraint force of the belt for that purpose.
In order to solve the problem of the diameter growth, there has been proposed a pneumatic tire in which the cord angle of the belt with respect to the tire circumferential direction is set large in a center portion in the tire width direction, set small in both end portions, and set at middle in an intermediate portion (Japanese Unexamined Patent Publication No. 2003-48111) . This is structured such as to enhance the constraint force by regulating the cord angle in the periphery of the end portion of the belt, based on the knowledge that the diameter growth is most remarkable in a shoulder portion.
However, in such the tire, it can not be said that a suppressing effect of the diameter growth is sufficient as mentioned below, and it is known that there is still room for improvement. In addition, a strain tends to be increased in the end portion of the belt in which the cord angle is comparatively small, and there is a tendency that a belt durability is lowered. Accordingly, there is desired a pneumatic tire which can achieve an excellent belt durability while further suppressing the diameter growth of the tire.